Fix dos 2723 and prevent further price related dos attacks

[Jim8y]

close neo-project/neo-vm#458

fix #2723

Dynamic opcode price strategy

            var borderMax = 1024 * 1024 / (OpCodePrices[opCode] == 0 ? 1 : OpCodePrices[opCode]);
            
            // Price increase fraction
            var fraction = 3;
            // if the opcode is used for too many times
            // 100% - 110% borderMax => (1 + fraction) * price
            // 110% - 120% borderMax => (1 + 2fraction) * price
            // ...
            // 180% - 190% borderMax => (1 + 9fraction)) * price
            // >190% borderMax, exception

[Jim8y]

I did not update src/neo/SmartContract/Helper.cs

cause ordinary operations are not likely to trigger the *10 price.....to be honest, i dont think anyone other than hackers would trigger *10 price

[erikzhang]

Why the UT fails?

[erikzhang]

I don't think this solves the problem. When the count of opcodes reaches borderMax, I can send another transaction.

[roman-khimov]

I don't think this solves the problem. When the count of opcodes reaches borderMax, I can send another transaction.

And several "problematic" opcodes can be hammered on even in a single transaction.

[Jim8y]

I don't think this solves the problem. When the count of opcodes reaches borderMax, I can send another transaction.

Reaching the BorderMax only takes very small amount of time. But yes, this can only prevent dos from a single transaction or some (a few) transactions.
If we wanna prevent DOS from block level once for all, we need to sum up the OpCodes cross transactions during preexecution.

[Jim8y]

And several "problematic" opcodes can be hammered on even in a single transaction.

The maxBorder is related to the price of the Opcode, if it takes longer to execute, then its maxBorder would relatively be smaller.

[erikzhang]

If we wanna prevent DOS from block level once for all, we need to sum up the OpCodes cross transactions during preexecution.

But this introduces non-deterministic prices.

[Jim8y]

But this introduces non-deterministic prices.

We just count the number of each opcode then.

[erikzhang]

I will close this pr since it doesn't solve the problem.

And neo-project/neo-vm#471 should fix #2723.